Fix some quadratic name lookup behavior #3579
Merged
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@LukeXuan reported that in files with a large number of top-level definitions, typechecking becomes quadratic.
One reason for this is because (surprisingly) we were resolving top-level names in a list (env.gamma_sig) even though we also have a hash map for lookups. Removing this linear scan eliminates one source of the quadratic behavior. I just removed the lookup in gamma_sig.
I also fixed a couple of quadratic list concatenations in SMTEncoding.Encode, though as @mtzguido pointed out, switching systematically to catenable lists will be better.